This invention relates to high energy density batteries which utilize nickel electrodes as the positive electrode and more particularly relates to pressed nickel electrodes for use in such batteries.
In nickel electrodes, the active electrode material may be nickel hydroxide or a berthollide nickel oxide represented by the formula NiO.sub.x. The electrical conductivity of such active electrode materials is sufficiently low to require that they be used in combination with conductive diluents or equivalent structure. Suitable conductive diluents include powdered carbon (including its graphite form) and nickel in flake or powder form.
In sintered porous electrodes, the sintered nickel plaque provides the desired internal conductivity (rather than a diluent). However, due to the cost of introducing the active electrode material into the plaque, efforts have been directed to two other nickel electrode structures, namely, pocket and pressed nickel electrodes. In pocket electrodes, it is customary to alternate layers of the active electrode material with layers of nickel flake (as described in U.S. Pat. No. 857,929), whereas in pressed nickel electrodes, or graphite powder is mixed with the active electrode material (as described in U.S. Pat. No. 3,023,259).
Although batteries employing presently available pressed nickel electrodes function satisfactorily, the amount of conductive diluent employed in these cells is such that the diluent constitutes a substantial percent by weight of the weight of the active electrode material. Because the volume of the nickel electrode in any given application is limited, the relatively high percentage of conductive diluent means that the performance of such batteries is significantly limited since, of course, the conductive diluent is not an active electrode material and does not directly contribute to battery output. Thus, it will be understood that it would be advantageous to be able to substantially reduce the volume of conductive diluent in pressed nickel electrodes while equaling or improving upon the discharge capabilities of presently available batteries utilizing such electrodes.
Upon repeated cyling, the carbonaceous conductive diluent material is partially oxidized leading to carbonate formation in the alkaline electrolyte. Since cell performance is adversely affected by carbonate in the electrolyte, it is desirable to minimize the carbon content in the electrode.